I've noticed over the last few months that the headlghts (actually all the lights) on my '89 760 turbo wagon (180,000 km) pulsate slightly when the car is at, or slightly above idle. I do not remember this always being the case.

The idle speed is correct and steady and the alternator belt is correctly tensioned. I measured the voltage at the battery terminals and from the positive battery terminal to chassis ground and it appears to be a steady 14.5V according to my cheap analogue meter. The pulsations appear to be about two or three per second. At speed the effect is not noticeable.

Any helpful pointers as to the cause of this or what to check for?

Thanks, Martin Kestle

/// I read your post and thought about it before I realized that all alternators pulse but what you are describing is failure of one of the three winding and or diode sets. It could be as simple as a broken wire at a point where the windings/diode join or a shorted winding. You are missing a complete set of pulses so there is a hole in the alternator output you can see in the headlights.

Replace your Motor Mounts there due. You probably have vibrating noise in the cabin at idle or stopped at lights\signs.

Voltage regulator going bad???

Could be. Any idea where the voltage regulator is located on this thing?

Your pulsating electrical problem is most likely caused by poor brush contact in the alternator, either due to worn out brushes or a groove worn into the slip rings. The brushes and regulator are combined in a pack mounted into the back of the alt. You can check the brushes for wear by removing the pack and measuring how far the brushes extend out of the holder (it's supposed to be something like .22" min, but don't quote me on that). You can replace the whole brush-regulator pack or buy just the brushes and solder them in yourself. If new brushes don't fix it then the slip rings are likely worn. You can take it to an alt/starter shop for repair or disassemble the alt yourself and carefully machine and polish the groove out of the skip rings.

Thanks very much, gentlemen; you've been a great help.